The shoulder is the most mobile joint in the human body. It has 180 degrees of motion in abduction and forward flexion and 360 degrees of motion in circumduction. The shoulder complex consists of three bones: the upper arm bone (humerus), collarbone (clavicle), and shoulder blade (scapula). These bones are held together by muscles, tendons, and ligaments.
The head of the humerus resembles a ball, which is seated in the glenoid fossa or cavity of the scapula. The structure of the shoulder ball and socket joint allows a wide range of movement of the arm in relation to the upper torso of the human body. The mobility is achieved by having fewer static restraints as compared to similar joints in other parts of a human body. Having fewer restraints means less stability, so the shoulder also has the least static stability of any joint in the human body.
Whereas static stability is provided by bone and non-elastic soft tissues, dynamic stability is provided by elastic and contractile musculotendinous structures. Compensation for reduced static stability in the shoulder is accomplished with enhanced dynamic stability. Unlike static stabilizers, which can provide only endpoint restrains, musculotendinous structures surround the shoulder and provide dynamic restraint throughout the entire range of motion. Without dynamic restraints, the stability of the shoulder has been compared to the instability of a golf ball resting on a golf tee.
Stability for the glenohumeral joint is provided by the muscles which cross the shoulder joint. The rotator cuff is a deep muscle group that holds the human head in the glenoid fossa, preventing instability and injurious movements of the humeral head in all directions. The shoulder has a wide range of motion. Circumduction is the arc of circular motion of the shoulder in the saggital plane. The shoulder is also capable of moving through an infinite number of planes of motion, which are perpendicular to the arc of circumduction and which pass through and contain the axis of circumduction.
Flexion identifies a movement whereby the humerus is brought forward beside the thorax. Extension defines a position in which the humerus is returned from any position of flexion to the anatomic position. Abduction defines a motion wherein the humerus moves laterally away from the body. If the person swings his arm sideways, the humerus moves upward as well as laterally to an extended vertical position beside the head. Adduction defines the motion in which the humerus is returned to the side of the body from 180 degrees of abduction. Rotation defines the motion of the shoulder body wherein the humerus turns medially or laterally on its long axis.
Muscles surrounding the shoulder can be exercised to protect the shoulder joint and increase the strength in the upper torso area. To enhance strength through all planes of motion, one must strengthen the moving muscles in all planes of motion. The strengthening exercise requires resistance in any plane of motion of the joint through a full 360-degree arc. It is well known that by moving resistance in different planes of motion of a muscle, different planes of fibers are employed to move the resistance, stimulating maximum strength gains within the same specific plane of fibers and motion exercised. Conventional exercise machines provide resistance for shoulder motion in only a few planes of motion. But because strength, stability, and performance of the shoulder are specifically enhanced only in the planes of motion trained, comprehensive strength training of the shoulder requires resistance exercise in many different planes of motion.
The present invention contemplates elimination of drawbacks associated with prior art exercise machines and provision of a multi-axis exercise machine for the upper torso that provides resistance exercise in an exponential number of planes.